On the stellar content of the open clusters Melotte 105, Hogg 15, Pismis 21 and Ruprecht 140

CCD observations in the B, V and I passbands have been used to generate col our-magnitude diagrams reaching down to V similar to 19 mag for two slightl y characterized (Melotte 105 and Hogg 15) and two almost unstudied (Pismis 21 and Ruprecht 140) open clusters. The sample consists of about 1300 stars observed in fields of about 4' x4'. Our analysis shows that neither Pismis 21 nor Ruprecht 140 are genuine open clusters since no dear main sequences or other meaningful features carl be seen in their colour-magnitude diagra ms, Melotte 105 and Hogg 15 are open clusters affected by E(B - V) = 0.42 /- 0.03 and 0.95 +/- 0.05. respectively. Their distances to the Sun have be en estimated as 2.2 +/- 0.3 and 2.6 +/- 0.08 kpc, respectively, while the c orresponding ages estimated from empirical isochrones fitted to the Main Se quence cluster members are similar to 350 Myr and 300 Myr, respectively. Th e present data are not consistent with the membership of the WN6 star HDE 3 11884 to Hogg 15.